1. Technical Field
The invention relates to a zoom lens system and an imaging apparatus suitable for use in a video camera, a digital still camera, a portable information terminal (PDA: Personal Digital Assistance), and the like.
2. Description of Related Art
In imaging apparatuses such as a digital still camera, as imaging devices such as CCD (Charge Coupled Device), and CMOS (Complementary Metal Oxide Semiconductor) have been downsized, it has been also required to downsize the imaging apparatus as a whole. To downsize the whole of the imaging apparatus, it is preferable to downsize the whole of the lens system (that is, it is preferable to reduce a total length and an outer diameter of the lens system).
JP 2006-235062 A (corresponding to US 2006/0193062 A), JP 2005-215385 A (corresponding to US 2005/0168832 A), Japanese Patent No. 3706783 (corresponding to U.S. Pat. No. 6,633,437) and Japanese Patent No. 3392881 describe a four-group configuration zoom lens system in which four lens groups having positive, negative, positive, and positive refractive powers, respectively, are arranged in order from the object side. In the zoom lens system according to these publications, during zooming, the aperture diaphragm is configured to move independently or to move with a part of the lens groups.
In the zoom lens system according to JP 2006-235062 A (corresponding to US 2006/0193062 A), the aperture diaphragm is configured to move independently of the adjacent lens groups during zooming. However, since a moving distance of the aperture diaphragm with respect to an image plane during the zooming is relatively large, a length of a moving mechanism of the aperture diaphragm increases, and thus it is difficult to achieve downsizing. Also, since a focal length of the first lens group is relatively large, a length of the whole lens system also increases, and thus it is difficult to achieve downsizing.
Furthermore, in the zoom lens system according to Japanese Patent No. 3706783 (corresponding to U.S. Pat. No. 6,633,437), the aperture diaphragm is configured to move together with the third lens group during the zooming. However, since a focal length of the third lens group is equivalent to that of the fourth lens group, the movement of the third lens group during the zooming increases, and thus it is difficult to achieve downsizing. Furthermore, since the focal length of the first lens group is relatively large, the length of the whole lens system also increases, and thus it is difficult to achieve downsizing.
Furthermore, in the zoom lens system according to JP 2005-215385 A (corresponding to US 2005/0168832 A) and Japanese Patent No. 3392881, during the zooming, the aperture diaphragm is moved so that a position of the aperture diaphragm at the telephoto end is closer to the image side than that at the wide-angle end. Hence, a distance between the first lens group and the aperture diaphragm increases at the telephoto end, and thus outer diameters of the lenses of the first lens group increase. Therefore, it is difficult to achieve downsizing.